Methods for estimating the direction of a source of a radio wave by detecting the arrival direction of the radio wave are conventionally proposed and also put to practical use. Simply, when visibility is secured between the source of a radio wave and an observation position, a direct radio wave (hereinafter referred to as “direct wave”) arrives from the source and there is no reflector that reflects the radio wave in neighborhood, the source can be said to exist in the direction from which the radio wave arrives.
However, in a multipath environment, radio waves arrive by repeating reflections and diffractions through buildings or the like in a complicated way. For this reason, even when a radio wave is originated from a single source, radio waves arrive at an observation point (receiving point) from a plurality of directions (multicarriers arrive), and therefore it is difficult to simply estimate the direction and position of the source from the arrival direction of the radio wave.
As an application example of an originating position estimating apparatus that estimates the position of the source based on the arrival direction of the radio wave, for example, such an apparatus may be mounted on a vehicle. In this case, the originating position estimating apparatus identifies the position of a collision avoidance target such as a pedestrian or the like by estimating the position of a transmitter carried by the pedestrian. Thus, even when the collision avoidance target such as pedestrian is located out of sight from the vehicle, it is assumed to be possible to prevent traffic accident by letting the driver recognize the presence of the pedestrian or the like or controlling the vehicle according to the position of the pedestrian or the like.
Patent literature 1 describes an apparatus that displays the arrival direction of a radio wave assuming that a pedestrian or the like is located in the arrival direction of a radio wave and thereby allows the driver to recognize the presence of the pedestrian. To be more specific, a synthesized image as shown in FIG. 1A is obtained by estimating the arrival direction of the radio wave, applying imaging processing to the incoming radio wave and synthesizing the image with a background image separately acquired by a camera or the like. This synthesized image is displayed on car navigation or the like and the driver looking at the synthesized image can visually recognize the position of a collision avoidance target.
However, although the radio wave arrival direction substantially coincides with the direction of the source (pedestrian or the like) in a good visibility environment, the radio wave arrival direction often does not coincide with the direction of the source (pedestrian or the like) in a multipath environment. That is, the source (pedestrian or the like) may not always be located in the radio wave arrival direction in the multipath environment.
For example, when there is a wall surface such as building B in FIG. 1B which becomes a cause of multipath, in addition to a diffracted wave, radio waves come from directions which have nothing to do with the position of a pedestrian. This makes it difficult to estimate the correct position of the pedestrian. Due to the existence of many reflectors such as buildings, a road environment is generally a multipath environment. Therefore, a technique is required which accurately estimates an originating position of a radio wave when multiplexed propagation waves arrive.
Patent literature 2 proposes an apparatus that accurately finds out the arrival direction of a radio wave from a mobile body that originates only a vertical polarized wave in a situation in which there are waves reflected by a mountain or the like. This apparatus includes an azimuth measuring receiver that measures electric field strength and azimuth of a vertical polarized wave front of a received wave, a horizontal polarized wave receiver that measures electric field strength and azimuth of a horizontal polarized wave front of the received wave and an azimuth correction circuit that compares a ratio of the electric field strength of the vertical polarized wave front to the electric field strength of the horizontal polarized wave front measured by each of the receivers with a prescribed value and thereby corrects the azimuth of the vertical polarized wave to be calculated.
In a direct wave directly arriving from an antenna for originating a vertical polarized wave, the electric field strength of the vertical polarized wave front is generally by far greater than the electric field strength of the horizontal polarized wave front. On the other hand, the polarized wave front of a radio wave arriving after being reflected by a mountain or the like is changed due to the reflection, and therefore the difference in electric field strength between the vertical polarized wave front and the horizontal polarized wave front becomes smaller. Since the electric field strength is displayed in decibel (dB), this difference is a ratio of electric field strength in the strict sense of the word.
Since a reflected wave has a smaller ratio of electric field strength than that of a direct wave, if the receiving side compares this ratio with a prescribed value, it is possible to determine whether the incoming radio wave is a direct wave (that is, the direction of the incoming wave is a correct direction) or a reflected wave (that is, the direction of the incoming wave is a false direction). Thus, Patent literature 2 describes the technique capable of accurately determining the direction of the source by eliminating influences of reflected waves.